Phenol is a weak acid. In water it partially dissociates, losing a proton from the -OH group to form the phenoxide ion:C6H5OH ⇌ C6H5O− + H+ Phenol is more acidic than alcohols such as ethanol, which hardly dissociate in water, but it is weaker than carboxylic acids.Reaction with sodium hydroxide
Phenol is acidic enough to be neutralised by a strong base. It reacts with aqueous sodium hydroxide to form sodium phenoxide and water, and dissolves:C6H5OH + NaOH → C6H5O−Na+ + H2O Ethanol does not react with sodium hydroxide in this way.No reaction with carbonates
Phenol is not acidic enough to react with carbonates such as sodium carbonate. Carboxylic acids do react, releasing carbon dioxide. This gives a test to tell them apart: when sodium carbonate is added, ethanoic acid fizzes as it releases carbon dioxide, but phenol does not.Worked example: find the pH of 0.10 mol dm−3 phenol, taking Ka = 1.3 × 10−10 mol dm−3.[H+] = √(Ka × [HA]) = √(1.3 × 10−11) = 3.6 × 10−6 mol dm−3 pH = −log(3.6 × 10−6) = 5.4
Phenol is acidic enough to be neutralised by a strong base. It reacts with aqueous sodium hydroxide to form sodium phenoxide and water, and dissolves:
Phenol is not acidic enough to react with carbonates such as sodium carbonate. Carboxylic acids do react, releasing carbon dioxide. This gives a test to tell them apart: when sodium carbonate is added, ethanoic acid fizzes as it releases carbon dioxide, but phenol does not.Worked example: find the pH of 0.10 mol dm−3 phenol, taking Ka = 1.3 × 10−10 mol dm−3.